1. Field of the Invention
The present invention relates to an optical directional coupler.
2. Description of the Related Art
There has conventionally been provided an optical directional coupler in the field of optical communication. An optical directional coupler refers to a power distributor that employs optical coupling of optical signals propagating in two adjacent waveguides.
An example of a configuration of this optical directional coupler is disclosed in the literature (An Introduction to Photonic Switching Fabrics, S. Hinton, Plenum, 1993, pp. 47-55).
The following will describe the configuration of a first prior-art optical directional coupler 100 with reference to FIG. 7. FIG. 7 is a plan view of the first directional coupler 100 as viewed from the upper side of the main surface of a clad layer. Because a waveguide (i.e., core) 102 is covered by a clad layer 104 and a substrate (not shown in FIG. 7), the waveguide 102 cannot directly be seen from the upper side of the main surface 104A of the clad layer 104. In order to spotlight the shape of the waveguide 102, FIG. 7 shows the shape of the waveguide 102 (hatched portion) on this main surface 104A. The waveguide 102 comprises two waveguides, namely a first waveguide 106 and a second waveguide 108. The first waveguide 106 and the second waveguide 108 are disposed symetrically with respect to the centerline CL between the first waveguide 106 and the second waveguide 108, and include an optical signal input region R1O1, an optical signal coupling region R103 and an optical signal output region R105. In the optical signal coupling region R103, the first waveguide 106 and the second waveguide 108 are formed adjacently and are straight and parallel so that an optical field distribution of optical signals that propagate along the waveguides 106 and 108 couples substantially. In the optical signal input region R101 and the optical signal output region R105, the first waveguide 106 and the second waveguide 108 are formed with a certain angle cc to the centerline CL so that the optical field distribution of the optical signals does not couple. In short, the optical signal input region R101 and the optical signal output region RI05 are not coupling regions.
Next, the propagation form of an optical signal in the first directional coupler 100 is described below. This example is explained with reference to a case where an optical signal is input from outside to the first waveguide 106 of the optical signal input region R101. This optical signal propagates through the first waveguide 106 in the optical signal input region R101 and then enters the optical signal coupling region R103. In the optical signal coupling region R103, alternating power transfers between the first waveguide 106 and the second waveguide 108, the power of the optical signal propagating toward the optical signal output region R105. Then, the optical signal arrives at the optical signal output region R105. At this arrival, if the power of the optical signal is concentrated in the first waveguide 106, the optical signal propagates along the first waveguide 106 in the output region R105. If the power of the optical signal is concentrated in the second waveguide 108, the optical signal propagates along the second waveguide 108 in the output region R105. If the power of the optical signal is divided equally between the first waveguide 106 and the second waveguide 108, the divided optical signals propagate along the first waveguide 106 and the second waveguide 108, respectively.
An optical device, such as an optical switch or an directional coupler is made by employing the first optical directional coupler 100.
On the other hand, as a modification of the first optical directional coupler 100, there is provided a second prior-art optical directional coupler. FIG. 8 is a plan view as viewed from the upper side of the main surface of a clad layer of a second prior-art directional coupler 200. In the second directional coupler 200, a first waveguide 202 and a second waveguide 204 are disposed symmetrically with respect to the centerline between the first waveguide 202 and the second waveguide 204, and are curved smoothly (or to be differentiated). For example, when the second directional coupler 200 is used as an optical switch, an operation range (a permissible range of an applied voltage) in a bar state is wider than the operation range of the first directional coupler 100, thus, there is an advantage to perform the bar state easily.
However, for example, when the above-mentioned prior-art optical directional coupler is used in cascade-connection, the waveguide in the non-coupling region of optical signals or an adjacent region including the connection part is not needed to perform the function. The waveguide in the non-coupling region is used only for propagating optical signals, but does not contribute substantially to the coupling of optical signals. For existence of the waveguide in the non-coupling region, the whole length of the directional coupler in a cascade-connection, i.e. length along propagation direction of optical signals, cannot be shortened. Therefore, problematically, the optical device cannot be minimized.
Furthermore, for example, when a compound semiconductor is used as a material for a core (i.e. waveguide), the refractive index of the compound semiconductor is higher than a refractive index of a simple semiconductor. To generate optical coupling between two waveguides in an optical signal coupling region, the distance between the waveguides needs to be reduced. However, fine adjustment of the distance is difficult, which makes fine adjustment of the coupling coefficient (represented herein by symbol K) of the two waveguides also difficult. As a result, required coupling of the optical signals that propagate the two waveguides does not occur. Therefore, for example, when this directional coupler is used as an optical switch, cross-talk increases problematically. To guard against this, there has been need for an optical directional coupler that has a shorter whole length, and can be designed easily.
It is therefore an object of the invention to provide an optical directional coupler which solves the above described problem. According to the invention, the optical directional coupler includes a first waveguide, a second waveguide and a plurality of bridge waveguides connecting the first waveguide and the second waveguide.